Abstract
The effect of MgSO4 addition on alkali sulphates induced hot corrosion of a β-NiAl coating was studied by performing a series of tests with deposits of Na2SO4, Na2SO4-20 %K2SO4 and Na2SO4-12 %K2SO4-35 %MgSO4 (mol.% for all) at 700 °C in air with addition of SO2. For elucidating the observed corrosion phenomena, various salt-oxide/NiAl powder mixtures were exposed to the same hot corrosion test conditions. Two pseudo-binary phase diagrams of the Na, K, Mg-sulphate systems were calculated using an in-house developed thermodynamic database. Addition of MgSO4 to Na2SO4-20 %K2SO4 deposit salt inhibited the hot corrosion attack during 24 h exposure, which is related to consumption of aggressive K2SO4 by the formation of solid K2Mg2(SO4)3. However, extending the exposure with Na2SO4-12 %K2SO4-35 %MgSO4 deposit to 100 h resulted in severe corrosion attack. The latter observation is explained by a reaction between K2Mg2(SO4)3 and NiAl causing formation of a Mg, Ni and Al containing spinel accompanied by release of the aggressive K2SO4 into the liquid phase.
